It has long been known to permanently induce anaesthesia by the cutting of afferent nerves so that the patient no longer experiences pain or other sensation from that nerve related portion of the body. However, the physical resection or cutting of nerve tissue has the propensity to create various long term medical problems including the probable creation of neuroma and the also probable permanent undesired destruction of the nerve fibre or worse, the permanent unintended destruction of adjacent fibres and still other possible undesired medical outcomes. It has been known in more recent times to temporarily induce anaesthesia for a medium term period by causing heating (hyperthermia) of the identified nerve fibre transmitting the undesired sensation. A convenient and cost effective method is the application of controlled current which causes hyperthermia by electrical resistance heating performed at such frequency that no neurological response is stimulated and that this application of energy should be done under the further limitation of temperature excursion to limit the thermal damage to the structure of the subject nerve fibre. After a period of time with the nerve tissue maintained above a certain temperature the nerve tissue is said to “desiccate” and transmission of neural signals is disabled. This desiccation process, when carefully done, is also said to “denervate” the nerve fibre but leaves intact the inherent fibre structure. The recuperative powers of the body enable this residual nerve fibre to slowly re-canalise so that after a period of time, such as 12 to 24 months, the fibre is again active. The pain sensation may then return if the original cause of the pain has not healed or been successfully treated and if so the therapy can be repeated again without permanent effect.
This hyperthermal induction of anaesthesia has hitherto been carried out by inserting the monopolar needle like probes shown in FIG. 1 through the skin of the patient using expensive low energy X-Ray fluoroscopy and an Image Intensifier to guide the placement of the probe 1. Each insulated shaft 2 of such a probe 1 carries a single electrode 3 and the probes are energised with respect to the dispersive electrode 7 (also termed a base plate, return plate, ground plate or neutral electrode). The flow of current through the body tissue is schematically illustrated by a number of arrows in FIG. 1 and it will be seen that the highest current density is immediately adjacent the electrode surface or tip 3 The intention is that the current flowing through the tissue is concentrated around the single electrode 3 sufficiently to cause hyperthermia and diverges therefrom so as to disperse or diffuse towards the dispersive electrode 7. Such tissue volumes denervated in this manner are commonly very small and so require multiple probe placements to address a volume sufficient to enclose a suitable length of the nerve fibre and its probable position. By way of example, for each of the six zygopophyseal joints needed to be treated for cervical vertebrae pain (eg whiplash injury) up to nine such separate probe placements are typically necessary. The volume and shape of the area within which the monopolar RF current is sufficient around the needle to cause adequate heating for desiccation is indeterminate in shape due to both the non-homogenous tissue structure and the non-homogeneous electrical characteristics of the tissue. When multiple such indeterminate lesions are created manually side by side, not including most probable errors of positioning, there will be included within the boundaries of the intended lesion, interstitial volumes of non-desiccated tissue as shown in FIG. 1a. The overall lesion volume is thus said to be not “integrated” meaning it is not of one tissue type where the desired tissue type was desiccated tissue.
Further, the tissue of interest to pain therapists is often transmitted by neural tissue in close proximity to ligature and to bone as well as fatty deposits in and between septa (dividing walls) such as comprises tissue around the spinal column. This is clearly non-homogenous (in contrast to heart tissue, for example, which is relatively homogenous). Similarly, the most conductive tissue in the body is that around the bone (the periosteum). Thus electrical current (and in particular monopolar current which is inherently not specifically steered or directed) is consequently concentrated in this tissue selectively in relation to other higher impedance pathways by the driving electrical potential difference. As a consequence the monopole current which was intended to be diffuse away from the monopole electrode 3 often is not diffuse but is actually concentrated in the direction of the neutral electrode. The periosteum, or bone surface tissue, is thus often the first tissue type to be desiccated. This concentration of current in the anisotropic impedance structures surrounding needle placement is of particular concern to monopolar systems which has current migration to a neutral electrode of no significant placement in regard the desired shape of the lesion. The extra electrical current which might be used to create a larger lesion is consequently distorted and attendant heating in such tissue as the periosteum may possibly cause bone necrosis to occur in underlying bone tissue. In a similar fashion higher conductive vascular pathways through the bone will concentrate current pathways and add further risk. Further, periosteum or other unintended neural tissue obliteration distal to the intended region of desiccation by monopolar electrical current en route to the distant dispersive electrode 7 is undesirable and is a reason for caution among practitioners in the positioning of the needles 1 and the limiting of the amount of power applied in complex neural anatomy.
The current practice consequently uses the small monopole needle 1 to create small cylindrical volumes of desiccation immediately around the electrode 3. The established strategy is to increase the size of the desiccated volume by making repeated needle insertions which create small side by side cylinders of desiccated tissue leaving interstitial volumes of possibly viable neural tissue. Consequently the resulting desiccated volume is not an integrated or uniform whole so the duration of the intended anaesthesia is at least indeterminate. Furthermore, this sequential activity of multiple needle placement followed by electrode activation carries risk of structural damage to the targeted nerve fibre, is extremely time consuming and consequently costly. It has been persevered with as a therapy because of the significant need of chronic pain sufferers and the advantage that needles used in FIG. 1 are able to be passed though the skin and thus the procedure experienced by the patient is not unduly invasive and is a better therapy than surgery for reasons before mentioned.
It is also known to utilise radio frequency energy in the ablation (or tissue necrosing) of heart muscle (myocardium) during open heart surgery. Here the intention is to destroy tissue which is creating or propagating irregular muscle contraction and thereby bringing about an irregular heartbeat (arrhythmia). U.S. Pat. No. 6,096,035 (Sodhi et al) discloses a needle like probe used in such open heart surgery. The procedure in practice was to insert a fixed regular array or fixed matrix of such probes into the heart muscle during open heart surgery. The volumetric density of this matrix was sufficient in its relation to the size of the pathologic neuromuscular anatomy and each electrode was utilised to detect those muscle areas which were propagating the abnormal electrical potential depolarisation signals. Once these points of functional abnormality have been detected, radio frequency energy was then applied to the same detecting electrodes and those in a suitably sized surround so as to ablate or destroy this particular muscle tissue.
This situation is schematically illustrated in FIG. 2 in which a subset of three probes 10A, 10B and 10C are shown each having three spaced apart co-axial electrodes of cylindrical configuration. These three needle probes along with many others attached to the same manifold were inserted into the myocardium (heart muscle) which is substantially homogenous. The probes 10 were arranged with a fixed regular matrix like spacing because of their mechanical construct to the manifold. In the particular situation exemplified, abnormal electrical signals were detected from the electrodes in a “sensing” mode when no energy was being discharged by the device in the tissue.
As a consequence, when a radio frequency voltage was applied in a bipolar fashion sequentially activating all these electrodes, a portion of tissue was ablated or destroyed. No attempt was made to design or predetermine the shape of destroyed portion save to have it larger than the detected pathologic tissue.
The above described prior art application in relation to cardiac tissue is substantially different from the field of activity of the present invention in that, firstly open heart surgery is involved compared to a percutaneous acupuncture like insertion of needles in the field of the present invention. Secondly, the pathologic heart muscle tissue is intended to be ablated or destroyed then over time replaced with correctly functioning tissue whereas such tissue targeted by the subject device and means alters the nature of correctly functioning tissue and allows it to return to the same correctly functioning state subsequent to the period of anaesthesia. Thirdly, because the heart muscle itself generates electrical signals which move through such muscle tissue as a wavefront, it is not necessary or desirable for the needle like probes 10 to be positioned with demanding accuracy as in a comparative sense there are no neighbouring tissue structures which must be protected. This is because there is a sufficient density of probes and a sufficient number of electrodes in this fixed matrix arrangement for the defective region of muscular tissue to be electrically detected and then upon mechanical rearrangement of connections, be ablated.
Additionally no dynamic adaptation of the discharge pattern was undertaken nor are there any electrodes left out of what was determined to be the active set and indeed the only set to be activated for discharge of energy. The therapy was different and consequently the device contrived to provide such therapy was different.
U.S. Pat. No. 5,383,917 (Desai) discloses an alternative prior art technique involving ablation of heart muscle by passing a catheter including electrodes through a vein or artery and into the heart. Here the tissue to be ablated is immediately under the surface of the heart muscle (the endocardium). Although not as invasive as open heart surgery, the intention is to disable aberrant depolarisation pathways in the heart muscle underlying the placement of the electrodes. In such therapy the puncturing of the endocardium is not desirable given it would have to be done in the actively pumping and mobile heart wall. Again the tissue to be destroyed by ablation is substantially homogeneous. Furthermore, similarly, it is intended to cause destruction of the ablated tissue such that regrowth will replace it with correctly functioning tissue.
In inventions for the cardiac muscle ablation using RF energy and tumour tissue ablation where the shape of the ablated tissue volume is not intended to be controlled with any precision, it being intended that such volume simply contain the volume of pathologic tissue which is intended to be necrosed. Further, such ablation procedures do not usually need control of temperature and it is common to have the temperature uncontrolled which commonly exceeds 100 deg C. so creating vapour which transiently generates gaseous voids in the tissue. Such permanent necrosis and subsequent absorption by the body is undesirable for the purpose of neural anaesthesia. The fragile nature of the threadlike nerve fibres would be damaged by the explosive creation of vapour and the physical structure of the fibre is intended to be preserved for the re-establishment of the neural pathway and thus no resorption is desired.
In the case of myocardial tissue it is aberrant muscle tissue being ablated which is generating arrhythmias in heart muscle by circus loop depolarisation or as another example terminating fragmentary conduction across the AV septa which is in both instances pathologic tissue and in neither case is intended to revert to a conductive state. Whereas with the desiccation of neural fibre the neural fibre is usually not of pathologic tissue, but is transmitting undesired signals to the spinal column.